In recent years, it is becoming absolutely necessary for hand-held phones to have not only mere call-placing/receiving functions, but also a high-speed data communications function. Accordingly, demands for reduction in the areas/volumes of their high-frequency sections that implement communications are coming to be more stringent. Among these high-frequency sections, a filtering section, in particular, that sorts out necessary signals for communications is increasingly required to have more compact, higher-performance characteristics, along with the reduction in the dimensions of other high-frequency sections.
A so-called a resonator type filter that is such a combination of a serial arm resonator and a parallel arm resonator as disclosed in Patent Reference 1 listed below, is most commonly used as a means of realizing the high-performance filter required for mobile communications. In Patent Reference 1, a surface acoustic wave resonator that has roll-up window blind-shaped electrodes on a piezoelectric substrate and excites surface elastic waves is disclosed as a means of realizing a resonator. The resonator used in the resonator type filter disclosed in Patent Reference 1, however, is not limited to application as a surface acoustic wave resonator. For example, non-Patent Reference 1 listed below introduces examples of practical use of the FBAR (Filmed Bulk Acoustic Resonator) formed by depositing a piezoelectric thin film on a lower electrode and further depositing an upper electrode on the piezoelectric thin film to excite longitudinal vibration in the film-thickness direction of the piezoelectric thin film.
In contrast to this, the techniques outlined below are disclosed as publicly known means of improving the characteristics of a resonator type filter by connecting some high-frequency circuit element in parallel to a resonator.
Patent Reference 2 discloses a technique for improving frequency characteristics by connecting a capacitive element in parallel to a serial arm resonator and controlling the intervals between a serial resonance frequency and a parallel resonance frequency.
Patent Reference 3 discloses a technique for connecting a capacitive element in parallel to a serial arm resonator and thus conducting temperature characteristics compensations on this resonator according to the particular temperature characteristics of the capacitive element.
Furthermore, Patent Reference 4 discloses a technique for improving frequency characteristics by connecting an inductive element in parallel to a serial arm resonator and controlling the intervals between a serial resonance frequency and a parallel resonance frequency.
Besides, the techniques described in Patent References 5 and 6 are known as techniques for improving the filtering characteristics of a surface acoustic wave resonator.
The technique described in Patent Reference 5 is intended to improve the deterioration of the attenuation levels at the lower-frequency side by maintaining in a required relationship the resonance frequency and anti-resonance frequency of the composite resonator consisting of a series-connected acoustic wave resonator and a parallel-connected acoustic wave resonator.
The technique described in Patent Reference 6 relates to a filter which, in a parallel-arm acoustic wave resonator with a required resonance frequency and in a serial-arm acoustic wave resonator with an anti-resonance frequency, has an impedance element connected in series to the serial-arm acoustic wave resonator, and an impedance element connected in parallel thereto.
Patent Reference 1:
Japanese Patent Laid-Open No. Hei 11-195957
Patent Reference 2:
Japanese Patent Laid-Open No. 2001-345675
Patent Reference 3:
Japanese Patent Laid-Open No. 2001-44790
Patent Reference 4:
Japanese Patent Laid-Open No. 2004-242281
Patent Reference 5:
Japanese Patent Laid-Open No. 2003-347896
Patent Reference 6:
Japanese Patent Laid-Open No. 2002-223147
Non-Patent Reference 1:
ELECTRONICS LETTERS, 13th May, Vol. 35, No. 10, pages 794-795